A Mobile Assisted Handoff (MAHO) function is presently specified for use in the Dual Mode (analog and digital) cellular mobile telephone system. Specifically, the EIA/TIA Interim Standard entitled "Cellular System Dual-Mode Mobile Station-Base Station Compatibility Standard" IS-54-B (April 1992) specifies the MAHO function in Section 2.4.5. Briefly, the MAHO function requires a mobile station to furnish RF channel signal-quality information to a fixed site base station that serves the mobile station. There are two types of channels upon which the mobile station performs signal measurements: (a) a currently assigned forward digital traffic channel, and (b) any other specified forward RF channel(s). The digital traffic channel has a Time Division Multiple Access (TDMA) format having six time slots that occur within a 40 millisecond frame.
Reference is made to FIG. 1 wherein a first base station (BS1) has an associated RF coverage area that overlaps the RF coverage areas of base stations 2 and 3 (BS2 and BS3). Each base station transmits with a unique, predetermined channel frequency. The channel frequencies are spaced apart from one another by 30 kHz. In FIG. 1 a mobile station (MS) is assumed to be served by BS1. When the MAHO function is activated by the receipt of a command from BS1, the MS first measures the received signal strength (RSSI) and estimates the Bit Error Rate (BER) of the current forward digital traffic channel. The MS subsequently reports the estimated BER to the serving BS1 with a Channel Quality Message.
The MS also measures the RSSI of one entry of a received Measurement Order Channel List during an idle time slot at 20 millisecond (1/2 frame) intervals. By example, if the RF channels associated with BS2 and BS3 are included within the Measurement Order Channel List, the MS will retune its receiver during an idle time slot to the frequency of BS2, measure the RSSI, and then retune its receiver during another idle time slot to the frequency of BS3 and make another RSSI measurement. The resulting RSSI estimates are reported to the serving BS1 in increments of 2 dBm within a range of -113 dBm, or less, to -51 dBm or greater. The various measurements and estimates made by the MS are employed to determine a next base station to which the MS will be assigned during a handoff procedure.
However, a problem arises during the use of a conventional AC-coupled RF receiver. In the conventional receiver down conversion circuitry produces a base band signal which is sampled with an Analog to Digital (A/D) converter. The resultant base band signal has been found to contain a large amount of DC-power and other low frequency power spectrum components. Also, different base band units (filters and amplifiers) are AC-coupled together, with each unit typically being biased to a particular, different voltage reference.
When making a MAHO RSSI measurement, the receiver is required to tune from the assigned forward digital traffic channel to another forward RF channel, make the signal strength measurement, and then tune back to the assigned forward digital traffic channel, all with a period equal to one half of a frame (20 milliseconds). However, when switching between RF channels to make the MAHO RSSI measurements, a large DC-power in the base band signal will charge the AC capacitors located in the signal path, resulting in a shift in the internal voltage reference levels. The internal voltage reference levels will return to a nominal condition in accordance with an internal time constant. Unfortunately, the time constant is typically of a large enough duration that the original biasing levels will not be restored before the mobile telephone is required to receive in a next assigned time slot. As a result, and as is shown in FIG. 4, the receiver will operate incorrectly due to the presence of a "triangularly" shaped DC-component.
When performing the MAHO function the receiver should ideally be capable of measuring the power of the pure carrier (without any modulation). However, the conventional receiver described above will be blocked due to the large DC base band component. This may increase the BER and result in the mobile station losing synchronization with its assigned forward digital traffic channel. Also, the DC-power will tend to be filtered by the AC capacitors in the signal path, resulting in an inability to measure this power and in the generation of an erroneous RSSI estimate. An erroneous RSSI estimate, if reported to the base station, can result in the handoff of the mobile station to a less than optimum base station.
It is thus an object of this invention to provide a method for accurately measuring the signal strength of an RF channel.
It is a further object of this invention to provide a mobile telephone that operates to obtain an accurate RSSI estimation on an RF channel other than an assigned RF channel.